Recently, the development of electric vehicles and hybrid electric vehicles (hereafter, this is called simply “an electric vehicle and the like”) each carrying a battery for a motor drive has been advanced rapidly. In order to enhance steering characteristics and cruising range, naturally, light weight and a thin profile are requested also for a battery mounted in an electric vehicle and the like. In order to make a battery light and thin, a film covered battery has been developed using laminate material which is made into a film by superposing a metal layer, such as aluminum, and a thermal welding resin layer on the case body through an adhesive layer, and making them thin. Generally, the laminate material has a structure in which both surfaces of a thin metal layer, such as aluminum are coated, with a thin resin layer, and which is resistive to acid and alkali and which has lightweight and flexible property.
Generally, the characteristics of a battery are influenced by expansion and shrinkage of the active material of a positive electrode and a negative electrode at the time of discharge and charge. Thus, heretofore, deformation of a battery has been suppressed by using a metal case. Nevertheless, in the case of a film covered battery, it is hardly possible to suppress swelling of a battery that has a laminate film case. To suppress battery swelling, it is necessary to apply a proper load to the battery.
For this reason, in order to suppress the swelling of a battery, for example, an organic electrolyte secondary battery is disclosed (refer to Japanese Patent Laid-Open No. 10-012278). The organic electrolyte secondary battery has an electrode group in which a positive electrode and a negative electrode are stacked together with a separator. The stacked electrode group of the battery is packaged in a pressure member whose sectional shape is angular U-shaped. The angular U-shape has a plurality of protrusions respectively on the inside of both faces of U-shape and these faces are opposite to each other.
In addition, although a film covered battery is constructed by putting a electrical device element between laminate materials which face each other and by welding a peripheral portion of the laminate materials by thermal welding, a gas discharger is provided, in a part in a thermally fused portion of this film covered battery, through which peeling off is caused in a portion by pressure which is lower than in other portions.
When a voltage outside of a standard range is applied to a battery at the time of use of the battery, gas may be generated by electrolysis of an electrolyte solvent, and internal pressure of the battery may rise. Furthermore, even if a battery is used at high temperatures outside of a standard range, gas is generated by decomposition of electrolyte salt, and the like.
Fundamentally, although it is ideal to use a battery within a standard range and to prevent the generation of gas when a control circuit of a battery malfunctions due to a certain cause and an unusual voltage is applied, or when the temperature becomes unusually high due to a certain cause, depending on the case, a lot of gas may be generated.
Such generation of a gas inside a battery causes the internal pressure of a battery to rise. Many batteries that use a metal can as a case to prevent accidental battery discharge resulting from extremely internal pressure have a pressure safety valve which allows gas to escape to the outside when the internal pressure of the battery rises. However, it is difficult structurally to provide a pressure safety valve in a film covered battery which uses a film as a case. Although film expands in a battery case that uses a film as a cover, the case member will eventually explode and gas will be vented from the location where excessive internal pressure has built up, and because it is not possible to specify where a rupture occur, the rupture may adversely affect the surrounding apparatus and the like depending on the location where the gas is vented.
Then, in a film covered battery, in order prevent a malfunction by such gas generation inside the battery, a gas discharge portion which is mentioned above is provided in a thermally fused portion.